Drills for originating cylindrical holes in a work piece have long been known. Many types of rigid machine controlled drilling operations, such as drill presses, produce acceptable results. However, hand-held or less than rigid drilling operations often produce sloppy, oversized, oblong or oval holes. This is especially troublesome with respect to drilling metals.
Historically, this has resulted in the need for first drilling a smaller hole and then using a precision reamer to complete a straight, precise, cylindrical hole. This two-step process can be cumbersome and time consuming, not to mention adversely affecting drill bit life due to friction heat build up on the cutting edges.
Hand-held or less than rigid drilling operations are further complicated by operator errors and slight changes of angle during the drilling process of a single hole. If this occurs, the cutting edges of the margin adjacent the flutes will cut into the sidewalls of the hole created by the cutting tip of the drill bit because the standard margin runs the length of the flutes. This results in improper removal of material and an oblong or oval hole.
The margin or major diameter of the drill bit determines the size of the hole based on the relative center of the chisel point and equal length of the cutting lips. The margin's benefit is in creating the diameter only at the end of the drill where it joins the cutting lip. If the margin extends the full length of the flute, as it does in common drill bits, its usefulness reverses and becomes detrimental to the precision hole created by the cutting lips. Transverse pressure and changes of angle allow the trailing margins to contact and cut into the sidewalls of the drilled hole.